It is common practice to use a mass damper to improve the ride in a vehicle. Mass dampers are attached to the vehicle frame and tuned to resonate at a predetermined frequency in order to counter vehicle vibrations. Pick-up trucks tend to have higher sensitivity to vertical shake due to the lower resonance frequency of their frame and rear axle modes. Many pickup trucks exhibit uncomfortable vertical ride vibrations induced by road inputs. These high levels of vibration are induced by an interaction between the vehicle's first bending mode and rear axle hop mode. Conventionally, spare tire assemblies in trucks and SUVs are rigidly attached to the undercarriage of the vehicle. Some damper designs, however, include the use of an apparatus for mounting a spare tire to the vehicle in a manner to use the tire as a mass damper.
For example, PCT Application No.: WO 2006/044943 titled “Spare Tire Mass Damper” teaches the use of a damper assembly having torsional isolators. The spare tire is pivotally mounted with respect to the vehicle frame and the spare tire resonates about one end of the damper assembly. Damper assemblies that use springs, however, are limited in that the springs provide a constant stiffness, thus resulting in constant damper tuning frequency. The springs cannot be adjusted to increase or decrease the damper frequency when it is desirable to do so. Accordingly, the damper assembly cannot be adjusted or tuned to meet various vehicle demands such as, e.g., payload variations, varying wheelbase lengths, or rougher terrain.
Therefore, there is a need for a vibration damper assembly that has isolators with adjustable frequency and/or stiffness so that the vehicle can be tuned for different conditions.